This invention relates generally to the field of sheet feeding, and more particularly to a method of controlling the operation of one or more sheet feeders to cause proper separating and of sheets from the one or more feeders so that they are injected into a common feed path in properly timed sequence with other sheets traveling in the common feed path.
The advent of high speed computer printing technology during the past decade or more has led to the development of an abundance of technologically complex related peripheral equipment for processing the output of most computer printers. One such process involves the printing by means of a high speed laser printer of a master document, such as a letter, each of which is individually addressed to a number of different individuals, typically a very large number, such as a thousand or more, in order to justify the cost of the equipment involved in the overall process. At least one, but more typically a plurality of other sheets of material, such as an advertising flyer, information sheet, pictorial material, etc., usually referred to as pre-print sheets, are associated with the letter in an accumulator to form a collation, which in turn is then fed to a folding machine which folds the letter, and the other sheets if need be, to fit into a mailing envelope. The folded collation is fed to an envelope inserting machine where it is inserted into a waiting envelope, which may then be closed, sealed and passed through a mailing machine for ultimate mailing. Although typical material associating processes would involve a plurality of pre-print sheets as just described, the principles of the invention are applicable to the addition of only one pre-print sheet to the letter, and the invention is hereinafter described and claimed to cover this arrangement. Also, it should be understood that the foregoing is only a representative sequence of operations which may be carried out in a particular installation, and one or more of the foregoing steps may be omitted, or other more specialized steps may be added, depending on the requirements of the particular sheet processing installation.
Since the material being assembled and inserted into the envelopes for mailing is addressed to specified individuals, it is very important, as a matter of business policy, to maintain the integrity of the collation by ensuring that it contains the correct number of sheets, and the only way to do that is to count the sheets of paper entering the accumulator where the collation is formed. To do this accurately, it is necessary to have a small gap between the trailing edge of a preceding sheet and the leading edge of a succeeding sheet, so that a suitable sensor can discern the end of one sheet and the beginning of the next, and do so with maximum accuracy while the sheets are traveling at a high rate of speed, in the order of 30 inches per second, which translates into slightly over two sheets per second with a two inch gap between sheets, or 1.17 seconds per sheet. If pre-print sheets are not fed accurately and the gap is eliminated, i.e., either a pre-print sheet is fed too fast and overlaps the trailing edge of a previous laser sheet, or is fed too slowly and overlaps the leading edge of a succeeding laser sheet, then individual sheets entering the accumulator cannot be counted accurately, thereby compromising the integrity of the collation, i.e., the mail piece, that is inserted into an envelope.
In a typical installation, the letters are printed and addressed in a high speed laser printer, which is a synchronous device capable of delivering a sheet every three and one half seconds. If it is desired to insert one or more pre-print sheets with each laser sheet, the pre-print sheet or sheets must be withdrawn from appropriate feeders in which it or they are stored and fed into a common feed path that directs it or them into the feed path leading from the laser printer to the accumulator, and done so with the aforementioned gap between each of the laser sheets and within the time available between delivery of successive sheets from the laser printer. Thus, the feeders for any pre-print sheets must feed these sheets with extreme accuracy and reliability in order to ensure that the pre-print sheets will always enter the feed path of the laser sheets in the precise timed relationship that is necessary for consistently proper accumulation prior to the final collation being folded and inserted into envelopes.
A major problem that exists with virtually all types of commercially available sheet feeders is that the rate of speed at which the primary sheet feeder, i.e., the feeder that separates the top sheet from the stack and feeds it out of the storage device, can feed a single sheet from a stack varies greatly (in terms of milliseconds, occasionally even seconds) depending upon various physical characteristics of the paper involved. For example, such characteristics as paper weight, thickness, static adhesion, surface finish and coefficient of friction, as well as external circumstances such as atmospheric humidity where the process is being carried out, can affect the speed at which paper can be fed from the same feeder. Even paper packaging techniques can have an adverse effect on uniformity of feeding, since in a ream of paper, depending on how it is packaged, it is impossible to guarantee that all adjacent sheets have uniform coefficient of friction. Since it is impossible as a practical matter either to always have the same paper and/or environmental conditions existent all the time, a given primary feeder cannot be relied upon to feed the sheets from the stack with the accuracy required for the application under consideration.
This problem has been partially solved in certain prior art applications by initially feeding a sheet from each pre-print sheet feeder a "staged" position in which the leading edge of the sheet has been engaged by a supplemental feeding device that is external to the storage device in which the stack of pre-sheets is stored. This effectively removes the pre-print sheet from the control of the initial sheet separator and feeder for the purpose of injecting the pre-print sheet into a feed path. The obvious advantage of this is that the accuracy of the supplemental feeding device is not dependent on variations in the aforementioned physical characteristics of the paper, nor variations in atmospheric conditions, with the result that the supplemental feeding device can generally be relied upon to inject a sheet into the common feed path for the laser sheets and the pre-print sheets once the supplemental feeding device is appropriately triggered to commence operation.
A major problem with this arrangement was that the primary feeders could not be relied upon to deliver only a single sheet to the supplemental feeding devices within the short time permitted by the rate of feed of the laser sheets over the range of paper and/or environmental characteristics which could be anticipated in typical applications of the type under consideration. For example, light weight paper with a smooth finish will feed accurately at a high speed, but that speed will produce doubling or tripling with heavier paper having a rough finish. With the primary feeder feeding at a slower speed to accommodate the heavier paper, the sheets may not reach the supplemental feeding device in time. So it becomes necessary to adjust the speed of the primary feeder for a specific type of paper, which prevents the feeding device from operating effectively with mixed paper, or with the same paper but at different times under varying atmospheric conditions. Thus, this solution has not been widely accepted.
Further development attempted to solve this problem by providing algorithms for the primary pre-print sheet feeder which could adjust the speed of operation of the feeder to accommodate differences in the rate of sheet feed dependent upon variations in physical and/or environmental characteristics. These algorithms adapt to paper over a stack, that is, a motor speed for the feeder is initially selected and held constant for a particular sheet. If the sheet is slow to stage, then the motor speed is changed for the subsequent sheet so that it will properly feed the next sheet and desirably all subsequent sheets. However, these algorithms have a number of disadvantages and drawbacks, one of which was that they cannot arrive at a desired motor speed for the subsequent sheets in the stack if there are sheets of different characteristics, because the algorithm is constantly adjusting the motor speed for a subsequent sheet each time it senses a sheet feed of a different speed. These algorithms also imply that the variance of stage times on the first few sheets will be quite high and this can cause problems in systems that have no tolerance. Still further, these algorithms also have a problem with disturbances, i.e., a sudden abnormal change in any normal operating condition, with the result that if, for example, a particular sheet is static charged, it may take longer to stage, and this would cause the controller to adjust incorrectly to subsequent sheets. The end result of these conditions is that there is still a degree of unpredictability about the rate of sheet feed for any given sheet in the stack, which can cause a sheet to stage beyond the period of time normally provided, thereby causing a mis-feed or a non-feed, depending on the nature of the microprocessor, and consequent improper accumulation.
Thus, there is still a need for an improved algorithm for the pre-print sheet feeders which virtually guarantees that the top pre-print sheet of a stack will stage at the proper time in order to ensure proper gap spacing between successive sheets in the flow of sheets to the accumulator, thereby permitting consistently accurate counting of the sheets to ensure the integrity of the final collation, and which also permits sheets of different physical characteristics to be placed in the stack without sacrificing the assurance that they will be staged properly.